[Foundation] URL: Fix availability of the new hash(into:) implementation

Author: lorentey

stdlib/public/Darwin/Foundation/URL.swift

@@ -626,7 +626,36 @@ public struct URL : ReferenceConvertible, Equatable {
         return _url.hash
     }
 
+    @_alwaysEmitIntoClient // Introduced in 5.1
     public func hash(into hasher: inout Hasher) {
+        // We expect this function to eventually satisfy the corresponding
+        // requirement when URL starts conforming to Hashable. However, in the
+        // meantime, it is useful to provide it so that user code can simply
+        // declare the conformance if desired, without implementing anything.
+        //
+        // We want this definition to be available to all clients, even those
+        // that get deployed with the 5.0 Foundation overlay where URL did not
+        // provide this method. The `@_alwaysEmitIntoClient` attribute above
+        // forces this function to get compiled into any user code that uses it.
+        if #available(macOS 9999, iOS 9999, tvOS 9999, watchOS 9999, *) {
+            // When we're on recent enough ABI, we forward the call to the
+            // resilient implementation provided below. The availability check
+            // above guarantees that the entry point will exist in the library
+            // we link with.
+            _hash(into: &hasher)
+        } else {
+            // This is the hash encoding that gets used with the 5.0 ABI. It has
+            // to match the version of == implemented by 5.0, so it shouldn't
+            // ever be changed.
+            hasher.combine(_bridgeToObjectiveC())
+        }
+    }
+
+    @usableFromInline
+    @available(macOS 9999, iOS 9999, tvOS 9999, watchOS 9999, *)
+    internal func _hash(into hasher: inout Hasher) {
+        // This code is beyond a resilience boundary. It has to be kept in
+        // sync with the definition of == implemented below.
         hasher.combine(_url)
     }